Aluminum (Al) is an attractive anode material for energy storage and conversion. Its relatively low atomic weight of 26.98 along with its trivalence give an electrochemical equivalent of 2.98 Ah/g, compared with 3.86 Ah/g for lithium, 2.20 Ah/g for magnesium, and 0.82 Ah/g for zinc. From a volume standpoint, aluminum should yield a capacity per unit volume of 8.04 Ah/cm3, compared with 2.06 Ah/cm3 for lithium, 5.85 Ah/cm3 for zinc, and 3.83 Ah/cm3 for magnesium. Aluminum-based batteries, in principle, have better chemical stability in many electrolytes than do lithium-based batteries. Additionally, aluminum is an abundant and relatively inexpensive metal.
The state of the art includes many Al battery chemistries based on aqueous electrolytes (see, for example, Li and Bjerrum, “Aluminum as anode for energy storage and conversion: a review,” Journal of Power Sources, vol. 110, pp. 1-10, 2002). However, for all aqueous electrolyte-based Al batteries, corrosion of the Al anode is a persistent problem. Corrosion of the anode reduces an Al battery's energy content and limits storage life. In addition, the corrosion products lead to passivating films that inhibit the battery reactions, thereby reducing the battery's power capabilities.
Due to the problems associated with aqueous electrolytes, it is beneficial to employ non-aqueous molten salt electrolytes in Al batteries. Compared with aqueous electrolytes, the advantages of molten salts are mainly three-fold: high electrical conductivity, fast electrode kinetics and hence less polarization, and high decomposition potential. Aluminum can be electrodeposited from non-aqueous media and therefore molten salt electrolytes are suitable for developing rechargeable aluminum batteries.
The state of the art includes Al batteries based on non-aqueous molten salt electrolytes that operate near room temperature (see, for example, Donahue et al., “Secondary aluminum-iron (III) chloride batteries with a low temperature molten salt electrolyte,” Journal of Applied Electrochemistry, vol. 22, pp. 230-234, 1992). The cathodes of these batteries contain transition-metal chlorides, such as FeCl3, which react according to one-electron reduction reactions (e.g., FeCl3+e−=FeCl2+Cl−). As a result of the atomic weight of chlorine and the stoichiometry of the one-electron reduction reaction, the theoretical active material specific energy density of these batteries is less than 350 Wh/kg. This theoretical energy content is too low for many applications.
In view of the current state of the art for Al batteries, there are several needs. First, Al batteries should effectively operate at or near room temperature. Second, to reduce or avoid corrosion, Al batteries containing non-aqueous electrolytes are preferred. Third, improved Al batteries preferably include a cathode that can be fully reduced upon battery discharge. Generally, there is a commercial desire for Al batteries with active material specific energy densities exceeding 400 Wh/kg.